The present invention relates to a laminated packaging material which is especially suitable for heat-sealable packages for liquid food, and which comprises a core of paper or cardboard, and oxygen and aroma proof barrier layers of ethylene-vinyl alcohol copolymer (EVOH) and polyamide (PA). Further, the invention relates to a manufacturing method of the same and to a sealed package for liquid food manufactured of the material, e.g. a milk or juice carton.
Laminated packaging materials used for liquid food packages and based on paper or cardboard are usually provided with one or more barrier layers, the purpose of which is to prevent the contamination of the packed product due to oxidation, and to preserve the product's taste and vitamins contained by it. Of polymeric materials, EVOH is known to have excellent barrier properties, but also polyamide has been used in liquid package cardboards because of its good barrier properties.
An example of the use of polyamide in a liquid package cardboard is described in the publication print FI 86610 and, correspondingly, the use of EVOH is described, for example, in the publication print FI 89567. In accordance with both the prints, the packaging material has been laminated on both sides with an external heat-sealing layer of LDPE (low-density polyethylene) so that the cardboard layer and the barrier layer of polyamide or EVOH is interposed between the heat-sealing layers. In the patent publication U.S. Pat. No. 4,701,360, there are known laminated packaging materials for liquid packages which may, besides an EVOH layer, include another oxygen proof barrier layer e.g. of polyamide. According to the publication, the EVOH layer is situated on the surface of the packaging material so that it also acts as a heat-sealing layer. In this case, EVOH is in contact with the packed liquid, which is a poorer solution with regard to the barrier properties of the material than the one in which the EVOH layer would be protected by a separate heat-sealing layer.
In the EP publication 0 318 771 there is disclosed a packaging laminate comprising EVOH and polyamide layers in combination with an aluminum foil. According to this publication the aluminum foil constitutes the main gas barrier of the laminate, and this barrier has been enhanced by an EVOH layer in order to compensate for small cracks that might occur in the foil. An optional polyamide (nylon) layer may be included to absorb moisture penetrating through said cracks. According to the teachings this layer is in general not necessary but preferred when the laminate is in danger of being subjected to strong external stresses, that is, in danger of developing cracks. If a polyamide layer is included it would have a thickness of 15-25 μm, preferably 20 μm. The aluminum foil in such a laminate forms an excellent oxygen barrier, but a drawback is the high cost of the foil.
The problem with EVOH in cardboards for liquid packages is that it is mechanically weak. For example, upon manufacture of carton-shaped packages, the material has to be folded, which easily causes cracks in the EVOH layer weakening its barrier properties. In addition, the EVOH layer tends to form holes in the area of its joints when the package is heat-sealed. This is caused by steam pressure which tries to escape from the cardboard layer because of heat and which EVOH cannot retain. In this respect, polyamide is a better material than EVOH, because it is less vulnerable to cracking upon folding, and it can also absorb moisture released from the cardboard. Because the barrier properties of polyamide are not equal to those of EVOH, it cannot as such replace EVOH, but it has been tried to solve these problems by combining EVOH and polyamide in a packaging material. Thus, the publication print FI 96752 discloses a method for solving the problem related with the formation of holes by a suitable choice of material for the barrier layer, one alternative being a compound of EVOH and polyamide. The purpose is to retain the viscosity and strength of the barrier layer material in the heat-sealing temperature. Also the patent publication U.S. Pat. No. 4,977,004 disclosing a food package cardboard comprising two separate EVOH layers recommends the compounding of EVOH and polyamide to improve the viscosity of the layer. However, a drawback of these solutions is that the barrier properties of EVOH suffer due to the compounded polyamide. The best way to make the material oxygen and aroma proof is to include a continuous, substantially clean EVOH layer in the packaging material. In addition, it has been discovered that the compound of EVOH and polyamide is more difficult to extrude than pure polymeric materials, mainly due to the tendency of the compound to gel.